Steam quality at various locations in a steam turbine is an important parameter since it plays a large role in the turbine's mechanical performance. Steam quality can also be used for diagnostic purposes and malfunction detection.
Historically, calorimeters have been used to determine steam quality. Difficulties with obtaining proper isokinetic steam samples limited the practical use of this approach. Beginning in the early 1980s, the British CEGB developed an optical traversing probe which relied on the forward light-scattering characteristics of sub-micron sized water droplets to characterize both the size distribution and the concentration of these drops. Analysis techniques were developed to convert the optical data to the wetness fraction and enthalpy. The CEGB data reduction method was based on the Mie scattering theory, and the mathematics involved inversion of ill-conditioned matrices to deduce size distributions. Comparison of results from several LP traverse tests performed in parallel with heat rate tests by the CEGB were reportedly very good, with differences in LP cylinder efficiency on average less than one percent. During the past fifteen years, researchers in both Europe and Japan have published descriptions of various similar optical techniques for measurement of steam wetness based on light-scattering characteristics of the condensed aerosol.
U.S. Pat. No. 4,137,462 describes a probe for measuring steam quality. The light from a monochromatic light source is transmitted to a measuring zone by a fiber optic bundle. The light passes through the measuring zone and is reflected back to a photodetector. The photodetector measures the intensity of the light. The ratio of the light intensity to the light intensity for dry steam is a measure of moisture content.
U.S. Pat. No. 4,497,577 describes a steam wetness measuring apparatus which comprises a light source, a part defining a measuring space when the apparatus is inserted into a wet steam flow, an optical fiber bundle for transmitting the incident light beam from the light source to the measuring space, and optical fiber bundles for transmitting to a photoelectric converter element a plurality of scattered light beams of different scattering angles obtained by radiating an incident light beam on the measuring space.
The prior art wetness probes do not provide a forward light scattering design which employs light having a wide wavelength range, and do not measure scattered light intensity for different wavelengths.